Interferometry is the process of measuring interference fringe patterns between an object beam and a reference beam. The comparison of these two beams allows for a very accurate measurement of an object beam that has been deformed by some surface or object.
Grazing-incidence interferometers employ an object beam that reflects from the object at a high-incidence angle. Such interferometers can be very useful because most surfaces have a very strong specular reflection as the incidence angle approaches 90 degrees. This allows for the interferometric measurement of surfaces that are not polished, such as the back side of a silicon wafer.
A shortcoming of prior-art grazing-incidence interferometers, such as disclosed in U.S. Pat. No. 6,239,351, is that they require a reference surface from which a reference beam must reflect. The reference surface needs to be parallel to and in close proximity to the object. This makes measuring the opposite surfaces of a generally planar object problematic. In addition, the zero-diffracted-order light beam is not employed and so needs to be suppressed.